1. Field of the Invention
The invention relates to an apparatus testing resistance to cleavage of cardboard tubes, in particular for tubes intended to be used as winding formers used, for example, in the printing industry.
2. Description of Background and Material Information
It is known to wind sheets of paper on mandrels comprised of cardboard tubes. Bobbins of this kind are handled by machines adapted to grasp from the inside the ends of tubes serving as winding formers.
Likewise, the bobbins, particularly in the printing industry, are positioned on unwinding machines or rotating machines in order to unwind the bobbins for printing the sheet of paper and/or cutting it and/or folding it, etc. To this end, machines used for holding the bobbins comprise two arms which are introduced into the two ends of the cardboard tube used as a winding former and which keys the latter by means of expansion heads provided with jaws capable of projecting towards the outside. The above-mentioned expansion heads are mounted free to rotate, so that the bobbin can freely turn as it unwinds.
It is clear that the winding formers are submitted to very heavy stress, caused by the weight of the bobbin, the speed and duration of its unwinding, to the pressure exerted by the expansion heads and, sometimes, to the braking stresses of the expansion heads which are blocked in rotation, in case, for example, of an emergency stop when the bobbin has a high kinetic energy.
The features of the cardboard tubes selected to be used as a winding former are therefore important and, up to now, the material used, the thickness of the tubes and other manufacturing parameters are chosen in an empirical manner with as the only resistance test, the actual operating test, by starting with the empirical observation that a tube resisting beyond a given time, as a function of the weight of the bobbin, should resist the entire time it is used.
It has been observed that cardboard tubes being used as winding formers are destroyed by cleavage or unflattening of the material at the end of a length of time which is a function of the above-mentioned stresses and more particularly a function of the weight of the bobbin. One can in effect observe that as the bobbin rotates (generally very quickly), each section of the mandrel, which turns with said bobbin, is alternately submitted to very different forces, the crushing caused by the weight occurring only when said section is turned upward.
There are already machines for testing the resistance of tubes to compression, to bending, to hardness (by ballbearings for example). These tests are used but are very inadequate when it is a matter of determining the features of a tube intended to be used, in particular, as a winding former.
This is why the inventor imagined an apparatus intended to test the resistance to the cleavage of cardboard tubes which are, to this end, cut in the form of samples or test pieces.